As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for a secondary battery has also sharply increased as an energy source for the mobile devices. In addition, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV) and hybrid electric vehicles (HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuels.
Depending upon the kind of external devices in which the secondary battery is used, the secondary battery may be used in the form of a single battery cell or in the form of a battery module having a plurality of unit cells electrically connected to each other. For example, small-sized devices, such as mobile phones, can be operated for a predetermined period of time with the output and capacity of one battery cell. On the other hand, a battery module needs to be used in middle or large-sized devices, such as laptop computers, portable digital versatile disc (DVD) players, small-sized personal computers (PC), electric vehicles, and hybrid electric vehicles, because high output and large capacity are necessary for the middle or large-sized devices.
The battery module includes a plurality of unit cells electrically connected in series and/or in parallel to each other.
In connection with this case, referring to FIGS. 1 and 2, it can be seen that in a case in which a positive electrode terminal and a negative electrode terminal are electrically connected to each other using a connecting member having a clad metal structure, processability is lowered.
Specifically, a clad metal type bus bar 100 is configured to have a structure in which an upper connecting part 102 made of aluminum is stacked on a lower connecting part 101 made of copper, and then the upper connecting part 102 and the lower connecting part 101 are rolled. Consequently, it is easy to weld a positive electrode lead 110 made of the same material as the upper connecting part 102, i.e. aluminum, to the clad metal type bus bar 100. In the same manner, it is easy to weld a negative electrode lead (not shown) made of the same material as the lower connecting part 101, i.e. copper, to the clad metal type bus bar 100.
In a case in which both a negative electrode lead 210 and a positive electrode lead 220 are welded to a clad metal type bus bar 200 as shown in FIG. 2, however, it is necessary to bend the clad metal type bus bar 200.
When the negative electrode lead 210 and the positive electrode lead 220 are welded to the clad metal type bus bar 200 in a state in which the clad metal type bus bar 200 is not bent, welding between the same copper materials or between the same aluminum materials are easily carried out. In a case in which welding between dissimilar metal materials is carried out, however, a brittle fracture occurs due to a dissimilar metal compound generated during welding with the result that the welded portions have low resistance to vibration.
In addition, it is necessary to perform an additional bending process as described above, whereby processability is lowered. Furthermore, welding of the dissimilar metals lowers safety of the battery module.